Nelson, 80, helped pioneer the outlaw country genre and was inducted into the Country Music Hall of Fame in 1993. He received the Kennedy Center Honors in 1998. Notable songs include "On the Road Again," "Bloody Mary Morning" and "Good Hearted Woman." Nelson is also well-known for his political activism.

Haggard, 76, was also a key figure in the outlaw country movement. He was inducted into the Country Music Hall of Fame in 1994 and the Oklahoma Music Hall of Fame in 1997. Hit songs include "I'm a Lonesome Fugitive," "Okie from Muskogee" and "Carolyn."